Microwaveable sheet, and production method of microwaveable sheet

ABSTRACT

A microwaveable sheet ( 1 ) that prevents permeation of the oil and is superior in moisture permeability, and a production method thereof. The microwaveable sheet ( 1 ) has a paper substrate ( 2 ), a resin layer ( 3 ) laminated through applying a coating liquid of a resin composition on one face of the paper substrate ( 2 ), in which the resin layer ( 3 ) has a plurality of air holes ( 3   a ). The microwaveable sheet ( 1 ) preferably has a water vapor transmission rate of from 200 g/m 2 /24 hrs to 15,000 g/m 2 /24 hrs. The microwaveable sheet ( 1 ) may have air holes ( 3   a ) formed by applying a coating liquid on one face of the paper substrate ( 2 ) using a roll having an intaglio on the surface thereof, or may have air holes ( 3   a ) formed by applying a coating liquid on one face of the paper substrate ( 1 ) having a shape with fine irregularity.

TECHNICAL FIELD

The present invention relates to a microwaveable sheet, and a production method of a microwaveable sheet.

BACKGROUND ART

Conventionally, exemplary sheets for use in wrapping a variety of foods include oil resistant sheets that are capable of suitably wrapping oily foods such as deep fried foods. As such sheets, oil-resistant papers for wrapping foods containing a large amount of oil ingredients have been publicly known (Japanese Unexamined Patent Application, Publication No. 2006-183221).

The oil-resistant paper has: a paper support; and a resin layer containing a hydrogen-bondable resin and oil absorptive particles provided on at least one face of the paper support, whereby superior oil resistance is attained.

However, a resin layer is included for ensuring oil resistance in such an oil-resistant paper, and therefore moisture permeability is likely to be diminished due to the resin layer. Thus, when foods such as deep fried foods are wrapped, qualities of the wrapped foods may not be maintained after a lapse of a certain period of time following the wrapping of the foods as a result of dew condensation on the surface of the wrapped foods since water vapor within the space surrounded by the sheet is not released outside. Additionally, also in the case in which the foods are heated in a microwave oven while the state of being wrapped is maintained, water vapor within the space surrounded by the sheet fails to be released outside, and thus the moisture is attached to the surface of the foods, whereby the qualities of the foods may be deteriorated.

PRIOR ART DOCUMENTS

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2006-183221

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention was made in view of such circumstances, and it is an object of the invention to provide a microwaveable sheet that prevents permeation of the oil to the external face and that is superior in moisture permeability, and to provide a method for producing a microwaveable sheet.

Means for Solving the Problems

The microwaveable sheet according to the present invention made for solving the foregoing problems includes:

a paper substrate; and

a resin layer laminated through applying a coating liquid of a resin composition on one face of the paper substrate,

in which the resin layer has a plurality of air holes.

Since the microwaveable sheet has a resin layer on one face of a paper substrate, permeation of oil is less likely to occur. In addition, the resin layer includes a plurality of air holes properly formed by applying a coating liquid of a resin composition on one face of the paper substrate; therefore, water vapor can be suitably permeabilized by way of the air holes while the permeation of oil as described above is prevented. Since such a microwaveable sheet has superior moisture permeability while securing the oil resistance as described above, even in the case in which foods such as deep fried foods and hamburgers wrapped by the microwaveable sheet are subjected to a heat treatment in a microwave oven in the wrapped state, water vapor is released outside while the permeation of oil is prevented, and therefore, deterioration of the texture is less likely to be accompanied through inhibition of the attachment of the water droplets onto the surface of the foods.

In addition, the microwaveable sheet preferably has a water vapor transmission rate of no less than 200 g/m²/24 hrs and no greater than 15,000 g/m²/24 hrs. According to such a constitution, when foods wrapped by the microwaveable sheet are heated in a microwave oven or the like in the wrapped state, water vapor generated from the foods can be suitably released outside. Thus, deterioration of the texture is less likely to be accompanied even if the foods are cooked by heating in a microwave oven or the like.

Furthermore, according to the microwaveable sheet, it is preferred that air holes are formed on the one face of the paper substrate through applying the coating liquid with a roll having an intaglio on the surface thereof. By thus forming the air holes through applying the coating liquid with a roll having an intaglio, the air holes as desired can be easily and certainly formed on/through the resin layer. Thus, the microwaveable sheet can achieve superior moisture permeability, while the oil resistance is maintained.

According to the microwaveable sheet, the aforementioned air holes may be formed through applying the coating liquid on the one face of the paper substrate having a shape with fine irregularity. By thus applying the coating liquid on one face of the paper substrate having a shape with fine irregularity, the air holes as desired can be easily and certainly formed on/through the resin layer. Accordingly, the microwaveable sheet can achieve superior moisture permeability while the oil resistance is maintained.

Moreover, in the microwaveable sheet, an application amount of the resin composition is preferably no less than 0.01 g/m² and no greater than 20 g/m² on the basis of the solid content. Thus, the microwaveable sheet can achieve further superior moisture permeability while the oil resistance is sufficiently maintained.

The microwaveable sheet preferably has the resin layer laminated on both two faces of the paper substrate. Accordingly, the microwaveable sheet can have preferred oil resistance and moisture permeability by way of the resin layers of both two faces of the paper substrate.

Additionally, a principal component of the resin layer is preferably an acrylic resin, an olefin-derived resin, or a carbonyl group-containing resin. According to such a constitution, the oil resistance of the microwaveable sheet can be improved.

Moreover, the paper substrate is preferably an oil-resistant paper. When the paper substrate is an oil-resistant paper, the oil resistance of the microwaveable sheet can be improved.

It is preferred that the microwaveable sheet further has a printing portion provided on the one face of the paper substrate. When such a constitution is adopted, for example, varying printing for each type of the food wrapped is made, whereby the type of the food can be distinguished through viewing the external face of the microwaveable sheet.

Additionally, the microwaveable sheet preferably has oil shieldability from one side to another of no less than 1 hour. When such a constitution is adopted, permeation of the oil can be more effectively prevented.

In addition, the method for producing a microwaveable sheet according to the present invention includes the step of:

applying a coating liquid of a resin composition on one face of a paper substrate to form a resin layer,

wherein the resin layer is formed such that a plurality of air holes are provided on/through the resin layer.

According to such a production method, the microwaveable sheet aforementioned can be produced, and thus the advantages as aforementioned can be achieved. More specifically, the microwaveable sheet produced according to the production method, oily matter and/or moisture are/is less likely to be permeated from one side to another of the microwaveable sheet, and the water vapor is likely to be released outside from one side to another of the microwaveable sheet.

It is to be noted that the term “air hole” as referred to means fine pores formed on/through the resin layer. The term “water vapor transmission rate” as referred to means a value determined in accordance with an upright cup test (JIS Z0208). The term “oil shieldability” means a determined value of a time period required for passage of 3 g of salad oil through a resin layer after dropwise addition, under a condition of a temperature of 40° C.

Effects of the Invention

As explained in the foregoing, the microwaveable sheet according to the present invention prevents permeation of the oil to the external face, and is superior in moisture permeability. In addition, the method for producing the microwaveable sheet according to the present invention enables a microwaveable sheet that prevents permeation of the oil to the external face and that is superior in moisture permeability to be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross sectional view illustrating a microwaveable sheet according to a first embodiment of the present invention;

FIG. 2 shows a schematic explanatory view illustrating a method for producing the microwaveable sheet shown in FIG. 1;

FIG. 3 shows a schematic cross sectional view illustrating a microwaveable sheet according to a second embodiment of the present invention; and

FIG. 4 shows a schematic explanatory view illustrating a method for producing the microwaveable sheet shown in FIG. 3.

DESCRIPTION OF EMBODIMENTS

Hereinafter, modes for carrying out the invention will be explained with appropriate references to the drawings.

First Embodiment Microwaveable Sheet 1

As shown in FIG. 1, a microwaveable sheet 1 has a paper substrate 2, a resin layer 3 laminated on one face of the paper substrate 2, and a printing portion 4 formed on another face of the paper substrate 2.

Although the paper substrate 2 is not particularly limited, a paper substrate produced by paper making from a pulp slurry containing pulp fibers as a principal component is suitably used. Examples of such a paper substrate 2 include virgin white roll papers, glassine papers, paraffin papers, kraft papers, simili papers, thin papers, general quality papers, Japanese papers, and the like. Of these, virgin white roll papers are preferred.

The grammage of the paper substrate 2 is preferably no less than 15 g/m² and no greater than 60 g/m², more preferably no less than 20 g/m² and no greater than 50 g/m², and still more preferably no less than 25 g/m² and no greater than 30 g/m². When the grammage of the paper substrate 2 is greater than the above upper limit, the stiffness of the paper substrate 2 increases, whereby the paper substrate 2 is less likely to be deformed so as to meet the shape of the food to be wrapped, and thus direct wrapping of the food with the microwaveable sheet 1 may be difficult. On the other hand, when the grammage of the paper substrate 2 is less than the above lower limit, the strength is reduced, whereby the paper substrate 2 may be likely to be broken.

The paper substrate 2 may contain a fluorochemical additive in order to impart or improve the oil resistance. The fluorochemical additive is exemplified by a composition containing a compound that includes a perfluoroalkyl group having a carbon chain of 6 or less carbon atoms, and the like. In the compounds that include a perfluoroalkyl group having a carbon chain of 6 or less carbon atoms, a longer carbon chain is preferred in light of superior oil resistance, and compounds that include a perfluoroalkyl group having a carbon chain of 5 or 6 carbon atoms are more preferred. It is to be noted that the carbon chain of the perfluoroalkyl group having 7 or more carbon atoms is not preferred since environmental burden may be increased. The fluorochemical additive used may be a commercially available product, and examples of the commercially available product include trade name “AG-E060” manufactured by Asahi Glass Co., Ltd., and the like.

The content of the fluorochemical additive in the paper substrate 2 on the basis of the solid content is preferably no less than 0.001 g/m² and no greater than 1.0 g/m², and more preferably no less than 0.3 g/m² and no greater than 0.7 g/m². When the content of the fluorochemical additive in the paper substrate 2 is greater than the above upper limit, the stiffness of the paper substrate 2 increases, whereby wrapping to meet the shape of the food may be difficult. On the other hand, when the content of the fluorochemical additive in the paper substrate 2 is less than the above lower limit, sufficient oil resistance may not be attained.

As the method for allowing the fluorochemical additive to be contained in the paper substrate 2, for example, a method in which the paper substrate 2 is impregnated with a solution containing the fluorochemical additive, a method in which a coating liquid containing the fluorochemical additive is applied on at least one face of the paper substrate 2, and the like may be exemplified. In the case in which a coating liquid containing the fluorochemical additive is applied on the surface of the paper substrate 2, the application may be executed either by on-machine coating with a coater attached to a paper-making machine, or by off-machine coating with a coater provided separately from the paper-making machine.

Although the thickness of the paper substrate 2 is not particularly limited, the thickness is preferably no less than 0.03 mm and no greater than 0.1 mm, and more preferably no less than 0.04 mm and no greater than 0.08 mm. When the thickness of the paper substrate 2 is greater than the above upper limit, the stiffness of the microwaveable sheet 1 increases, whereby the microwaveable sheet 1 is less likely to be deformed so as to meet the shape of the food to be wrapped, and thus direct wrapping of the food may be difficult. On the other hand, when the thickness of the paper substrate 2 is less than the above lower limit, the strength of the microwaveable sheet is reduced, whereby the paper substrate 2 may be likely to be broken.

The resin layer 3 is formed by applying a coating liquid of a resin composition on one face of the paper substrate 2. As a principal component of the resin composition, an acrylic resin, an olefin-derived resin or a carbonyl group-containing resin may be employed.

Examples of the acrylic resin include acryl polymers, copolymers such as acryl-styrene copolymers, and the like. Specifically, acrylic copolymers prepared by copolymerizing styrene, a styrene derivative, acrylic acid (methacrylic acid), an acrylic acid alkyl ester such as methyl acrylate, ethyl acrylate or butyl acrylate, as well as a methacrylic acid alkyl ester such as methyl methacrylate, and the like; ethylene-acrylic acid copolymer water soluble salts such as an ethylene-acrylic acid copolymer sodium salt and an ethylene-acrylic acid copolymer ammonium salt; copolymers of acrylamide with acrylic acid (methacrylic acid); copolymers of acrylonitrile with acrylic acid (methacrylic acid), and the like may be exemplified.

Examples of the olefin-derived resin include low-density polyethylene, high-density polyethylene, polypropylene, ethylene-based copolymers, and the like.

Examples of the carbonyl group-containing resin include styrene-acrylic acid ester copolymers, epoxy ester resins, urethane resins, oil-modified phenol resins, and the like.

The styrene-acrylic acid ester copolymer is not particularly limited, and may be produced by a well-known emulsion polymerization method. In this regard, when an emulsion polymerization method is employed, a surfactant is preferably used in order to attain a certain hydrophilicity. Examples of the surfactant include anionic surfactants such as fatty acid soap, alkylsulfonic acid salts, alkylbenzenesulfonic acid salts, alkylsulfosuccinic acid salts and polyoxyethylene alkylsulfuric acid salts. The surfactant is preferably used in an amount of no less than 1 part by weight and no greater than 10 parts by weight with respect to 100 parts by weight of the monomer mixture.

As the epoxy ester resin, a variety of epoxy ester resins may be adopted, which may be obtained by a reaction of an epoxy resin with a compound that includes a carboxyl group to cause esterification accompanied by opening of the oxirane ring.

The urethane resin is not particularly limited, and, for example, a product of polymerization of polyisocyanate with polyol may be employed. Examples of the polyol include polyether polyols, polyester polyols, polylactone polyols, polycarbonate polyols, and the like.

The isocyanate is not particularly limited, and examples thereof which may be employed include: aliphatic diisocyanate compounds such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate; alicyclic diisocyanate compounds such as isophorone diisocyanate, hydrogenerated xylylene diisocyanate, 1,4-cyclohexane diisocyanate and 4,4′-dicyclohexylmethane diisocyanate; aromatic aliphatic diisocyanate compounds such as xylylene diisocyanate and tetramethylxylene diisocyanate; aromatic diisocyanate compounds such as toluylene diisocyanate and phenylmethane diisocyanate; modification products of the diisocyanates described above (carbodiimide-, uretdione- or uretonimine-containing modification products, etc.), and the like.

The polyether polyol is not particularly limited, and examples thereof which may be employed include polyethylene glycol, polypropylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and the like.

The polyester polyol is not particularly limited, and examples thereof which may be employed include polyethylene adipate, polybutylene adipate, polyneopentylene adipate, poly-3-methylpentylene adipate, polyethylene/butylene adipate, polyneopentylene/hexylene adipate, and the like.

The polylactone polyol is not particularly limited, and examples thereof which may be employed include polycaprolactone diol, poly-ω-hydroxycaproic acid polyol, and the like.

The polycarbonate polyol is not particularly limited, and examples thereof which may be employed include products obtained by a reaction of: a diol such as propanediol-(1,3), butanediol-(1,4), hexanediol-(1,6), diethylene glycol, triethylene glycol or tetraethylene glycol; phosgene; and a diaryl carbonate such as diphenyl carbonate, or a cyclic carbonate such as ethylene carbonate or propylene carbonate, and the like. In addition, the polycarbonate polyol is exemplified by polyester carbonates obtained by a reaction of: a polyester or polylactone; phosgene; and a diaryl carbonate or a cyclic carbonate.

The oil-modified phenol resin is not particularly limited, and examples thereof which may be employed include resins obtained by allowing a phenol, an aldehyde and an oil to be reacted in the presence of an acidic catalyst.

The oil for use in the modification is not particularly limited, and examples thereof which may be employed include at least one selected from plant oils, particularly, cashew nut oil, linseed oil, wood oil, castor oil and tall oil. Of these, cashew nut oil is preferred in light of the storage stability. Moreover, as the oil-modified phenol resin, a cashew nut oil-modified novolak-type phenol resin is preferably employed.

The resin layer 3 may contain a filler such that the resin layer 3 has flat and smooth feature with respect to foods (i.e., an effect of preventing the resin layer 3 from attaching to the foods). The filler which may be contained in the resin layer 3 is not particularly limited, and exemplified by inorganic fine particles and organic fine particles. As the filler contained in the resin layer 3, aluminum silicate is suitably used, alternatively, silica, barium sulfate, titanium oxide and the like may be also used. The filler may be used either alone of one type, or two or more types may be used as a mixture.

The mean particle size of the filler is not particularly limited, and is preferably no less than 50 nm and no greater than 500 nm. The upper limit of the mean particle size of the filler is more preferably 350 nm, and still more preferably 300 nm. The lower limit of the mean particle size of the filler is more preferably 60 nm, and still more preferably 80 nm. When the mean particle size of the filler is greater than the above upper limit, the flat and smooth feature of the resin layer 3 with respect to foods may not be satisfactorily improved. To the contrary, when the mean particle size of the filler is less than the above lower limit, the filler is aggregated within the coating liquid, and thus the flat and smooth feature of the resin layer 3 with respect to foods may not be sufficiently exhibited.

The content of the filler in the resin layer 3 is not particularly limited, and is preferably no less than 1 part by weight and no greater than 20 parts by weight with respect to 100 parts by weight of the resin contained as a principal component of the resin composition. The upper limit of the content of the filler is more preferably 15 parts by weight, and still more preferably 10 parts by weight. Further, the lower limit of the content of the filler is more preferably 3 parts by weight, and still more preferably 5 parts by weight. When the content of the filler is greater than the above upper limit, the flat and smooth feature of the resin layer 3 with respect to foods may not be satisfactorily improved. To the contrary, when the content of the filler is less than the above lower limit, the condensity of the filler decreases, and thus the flat and smooth feature of the resin layer 3 with respect to foods may not be sufficiently exhibited.

The average thickness of the resin layer 3 is not particularly limited, and is preferably no less than 0.01 mm and no greater than 0.08 mm, and more preferably no less than 0.03 mm and no greater than 0.05 mm. When the thickness of the resin layer 3 is less than the above lower limit, the oil resistance of the microwaveable sheet 1 may not be sufficiently ensured. Whereas, when the thickness of the resin layer 3 is greater than the above upper limit, the stiffness of the microwaveable sheet 1 increases, whereby the microwaveable sheet 1 is less likely to be deformed so as to meet the shape of the food to be wrapped, and thus direct wrapping of the food may be difficult.

Additionally, the resin layer 3 has a plurality of air holes 3 a formed. It is to be noted that the confirmation of the air holes 3 a is not particularly limited, and a well-known method may be adopted for the confirmation. Also, the air holes 3 a may be confirmed through applying a pigment on the resin layer 3.

The printing portion 4 is not particularly limited, and a portion including a pigment fixed on the paper substrate 2 by a binder may be employed. The pigment is not particularly limited, and a black pigment, a white pigment, a red pigment, a blue pigment, a yellow pigment, a green pigment, an orange pigment, a purple pigment or the like may be appropriately used. Further, the binder is not particularly limited, and one type, or a mixture, a copolymer or the like of two or more types of a variety of synthetic resins such as an acrylic resin, a styrene-based resin, a polyester-based resin, a urethane-based resin, a polyvinyl-based resin, an alkyd-based resin, a petroleum-based resin, a ketone resin, an epoxy-based resin, a melamine-based resin, a fluorine-containing resin, a silicone-based resin, a cellulose derivative and a rubber-based resin may be employed.

The water vapor transmission rate of the microwaveable sheet 1 is not particularly limited, and is preferably no less than 200 g/m²/24 hrs and no greater than 15,000 g/m²/24 hrs. The upper limit of the water vapor transmission rate of the microwaveable sheet 1 is more preferably 13,000 g/m², and still more preferably 11,000 g/m². Moreover, the lower limit of the water vapor transmission rate of the microwaveable sheet 1 is more preferably 1,000 g/m², and still more preferably 4,000 g/m². When the water vapor transmission rate falls within the above range, in heating the food wrapped by the microwaveable sheet 1, in the wrapped state, in a microwave oven or the like, the water vapor generated from the food can be suitably released outside, and thus deterioration of the texture is less likely to be accompanied even if a heat treatment is carried out in a microwave oven or the like.

The oil shieldability of the microwaveable sheet 1 therethrough (from one side to another) is not particularly limited, and is preferably no less than 1 hour, more preferably no less than 1.5 hours, and still more preferably no less than 2 hours. Accordingly, permeation of the oil can be effectively prevented.

Production Method of Microwaveable Sheet 1

The production method of the microwaveable sheet 1 includes a resin layer-forming step, and a printing step.

According to the resin layer-forming step, the resin layer 3 is formed on one face of the paper substrate 2. In the resin layer-forming step, a coating liquid of a resin composition is applied on one face of the paper substrate 2, and the applied coating liquid is hardened with drying to form the resin layer 3.

The coating liquid is prepared by mixing the resin composition and a solvent. For example, the coating liquid is prepared by mixing the resin composition of a carbonyl group-containing resin or the like, with a water-based solvent. Although the water-based solvent is not particularly limited, it may be, for example, water alone, or a mixture of an alcohol such as ethanol or propanol with water, and the like.

The application method onto the paper substrate 2 may be carried out with a roll having an intaglio on the surface thereof. Thus, desired air holes 3 a can be formed easily and certainly on/through the resin layer 3. As a specific application method, a gravure coating method by a gravure printer may be exemplified. As shown in FIG. 2, the surface of a gravure roll 14 is has a plurality of recessed cells 14 a for the purpose of retaining a large amount of a coating liquid 13 of the material for forming the resin layer 3. Thus, by rotating the gravure roll 14 in a vessel 15 containing the coating liquid 13 so as to bring the gravure roll 14 into contact with the surface of the paper substrate 2, a large amount of the coating liquid 13 is held onto sites corresponding to the cell 14 a portions on the gravure roll 14, whereas a small amount of the coating liquid 13 is applied onto sites other than those corresponding to the cell 14 a portions. Accordingly, suitable air holes 3 a are likely to be formed at the sites other than those corresponding to the cell 14 a portions (i.e., sites where a small amount of the coating liquid 13 was applied). It is to be noted that before the gravure roll 14 is brought into contact with the surface of the paper substrate 2, a desired amount of the coating liquid 13 attached can be appropriately adjusted by dabbing a doctor blade (not shown in the Figure) on the surface of the gravure roll 14. Alternatively, a desired amount of the coating liquid 13 attached can be also adjusted by regulating the speed of rotation of the gravure roll 14.

The average depth of the recessed part of the cell 14 a of the gravure roll 14 is not particularly limited, and is preferably no less than 10 μm and no greater than 70 μm and more preferably no less than 30 μm and no greater than 50 μm. When the average depth of the recessed part of the cell 14 a is less than the above lower limit, a large number of the air holes 3 a of the resin layer 3 may be formed, whereby the oil resistance of the microwaveable sheet 1 is likely to be deteriorated. Whereas, when the average depth is greater than the above upper limit, the thickness of the resin layer 3 may be so great that the stiffness of the microwaveable sheet 1 increases, whereby direct wrapping of the food with the microwaveable sheet 1 may be difficult.

The application amount on the basis of the solid content is preferably no less than 0.01 g/m² and no greater than 20 g/m², and more preferably no less than 3 g/m² and no greater than 8 g/m². When the application amount falls within the above range, the microwaveable sheet 1 can achieve further superior moisture permeability while the oil resistance is sufficiently maintained.

The hardening with drying of the coating liquid is not particularly limited, and the coating liquid may be hardened with drying using a drying apparatus (not shown in the Figure).

According to the printing step, characters, pattern designs and the like for decoration are printed on one face of the paper substrate 2. In the printing step, printing portions 4 are provided on the face on the other side of the face of the paper substrate 2 on which the resin layer 3 was laminated in the resin layer-forming step. In this step, a variety of well-known methods may be employed as for the printing method. The printing method may be exemplified by a gravure printing method, an offset lithography method, a flexography method, a screen printing method, and the like.

Advantages

Since the microwaveable sheet 1 has the resin layer 3 on one face of the paper substrate 2, when foods are wrapped such that the resin layer 3 is brought to be the inner side (side facing to the food), permeation of oil exuded from the foods to the external side of the microwaveable sheet 1 can be certainly prevented by the resin layer 3. In addition, since the resin layer 3 includes a plurality of air holes 3 a, water vapor is suitably released outside through the air holes 3 a. Thus, even if foods such as deep fried foods and hamburgers are subjected to a heat treatment in a microwave oven in the wrapped state, the water vapor is suitably released outside from the air holes 3 a while the permeation of oil is prevented; therefore, deterioration of the texture is less likely to be accompanied through inhibition of the attachment of the water droplets onto the surface of the foods.

In addition, since the principal component of the resin layer 3 is an acrylic resin, an olefin-derived resin, or a carbonyl group-containing resin, the oil resistance of the microwaveable sheet 1 can be improved.

Still further, according to the microwaveable sheet 1, the air holes 3 a that suitably release water vapor outside the microwaveable sheet 1 can be easily and certainly formed by applying the coating liquid 13 with a roll having an intaglio on the surface thereof. Thus, the microwaveable sheet 1 can achieve superior moisture permeability while the oil resistance is maintained.

Additionally, since the microwaveable sheet 1 has the printing portion 4 on another face of the paper substrate 2, for example, when varying printing for each type of the food wrapped is made, the type of the food can be distinguished through merely viewing the external side of the microwaveable sheet 1.

Second Embodiment

Next, a microwaveable sheet 21 according to the second embodiment of the present invention is explained. It is to be noted that in the explanation of the second embodiment, regarding any member, etc., having an identical constitution or identical function to that in the first embodiment, detailed explanation thereof may be omitted through referring to the identical reference symbol.

The microwaveable sheet 21 according to the second embodiment is, as shown in FIG. 3, has a paper substrate 22, a resin layer 23 laminated on one face of the paper substrate 22, and a printing portion 4 formed on another face of the paper substrate 22.

One face of the paper substrate 22 employed has a shape with fine irregularity. The surface roughness resulting from the shape with fine irregularity of the paper substrate 22 (average roughness of the measurements at ten points) is not particularly limited, and is preferably no less than 0.01 mm and no greater than 0.05 mm and more preferably no less than 0.02 mm and no greater than 0.03 mm. When the surface roughness of the paper substrate 22 does not fall within the above range, the air holes 23 a as desired may not be properly formed on/through the resin layer 23 of the microwaveable sheet 21.

It is to be noted that as the paper substrate 22, similar ones to those for use in the first embodiment such as virgin white roll papers, glassine papers, paraffin papers, kraft papers, simili papers, thin papers, general quality papers, Japanese papers and the like may be used, which may also have similar grammage and thickness.

The resin layer 23 is formed on the face having a shape with fine irregularity of the paper substrate 22. In addition, the resin layer 23 has a plurality of air holes 23 a formed thereon.

Furthermore, the average depth in the irregularity having the air holes 23 a of the resin layer 23 is preferably similar to the average depth of the air holes 3 a of the resin layer 3 in the first embodiment.

As the material for forming the resin layer 23, similar materials for use in forming the resin layer 3 employed in the first embodiment may be used.

Next, a production method of the microwaveable sheet 21 according to the second embodiment of the present invention is explained with references to FIG. 4.

The production method of the microwaveable sheet 21 includes, similarly to the first embodiment, a resin layer-forming step and a printing step.

In the resin layer-forming step, a coating liquid 13 is applied on one face of the paper substrate 22 to form the resin layer 23.

The application method which may be employed includes a roll coating method carried out using a roll having a smooth surface, as well as a bar application method, and the like. Specifically, as shown in FIG. 4, by rotating a solid roll 34 in the vessel 15 containing the coating liquid 13, the coating liquid 13 is held onto the solid roll 34. Then, pressing the solid roll 34 against the face having irregularity of the paper substrate 22 enables the coating liquid 13 to be applied onto the paper substrate 22.

Also, when the solid roll 34 is pressed against the face having irregularity of the paper substrate 22, the pressure is adjusted in bringing the solid roll 34 into contact while the solid roll 34 is pressed against the resin layer 23, such that a thin resin layer is formed on the protruding parts of the face having irregularity of the resin layer 23. Thus, the resin layer is formed on the surface of the paper substrate 22 such that the resin layer 23 is formed to have a constant thickness with the smooth solid roll; therefore, appropriate air holes 23 a are likely to be formed at the parts on which the thin resin layer 23 was formed (top parts of the protruding parts of the paper substrate).

Furthermore, in the printing step, the printing portion 4 is formed on one face of the paper substrate 22, similarly to the step in the first embodiment.

In the microwaveable sheet 21 according to the second embodiment, the resin layer 23 is formed on one face of the paper substrate 22 having a shape with fine irregularity, and the air holes 23 a are likely to be appropriately formed at sites on which a thin resin layer 23 was formed (top portions of the protruding parts of the paper substrate 22 having a shape with fine irregularity. Therefore, water vapor is suitably released outside by way of the air holes 23 a. Therefore, since the microwaveable sheet 21 has superior moisture permeability while the oil resistance is secured, even in the case of heating in a microwave oven, the water vapor is suitably released outside by way of the air holes 23 a, and thus deterioration of the texture is less likely to be accompanied.

Other Embodiment

The present invention may be appropriately altered without departing from the scope intended by the present invention, without any limitation to the foregoing embodiments.

In the above embodiments, the explanations have been made on modes in which the resin layer is laminated on one face of the paper substrate; however, in the present invention, the resin layer may be laminated on both two faces of the paper substrate. Accordingly, the water vapor transmission rate as well as the oil resistance and the moisture permeability can be easily controlled.

Also, the application method on the paper substrate is not limited to the method aforementioned, but well-known methods such as a knife coating method, a blade coating method, a die coating method, a flexo coating method and a dipping method may be employed. When these methods are employed, the resin layer having desired air holes can be formed by, for example, adjusting the application amount of the coating liquid of the material for forming the resin layer.

Alternatively, the resin layer having desired air holes may be formed by applying on the paper substrate, the coating liquid of the material for forming the resin layer, in which the coating liquid contains a filler. The filler which may be contained in the coating liquid is not particularly limited, and inorganic fine particles and organic fine particles may be exemplified. As the filler which may be contained in the resin layer 3, aluminum silicate is suitably used, or silica, barium sulfate, titanium oxide, etc., can be also used. The filler may be used either one type alone, or two or more types may be used as a mixture.

The mean particle size of the filler is not particularly limited, and is preferably no less than 50 nm and no greater than 500 nm. The upper limit of the mean particle size of the filler is more preferably 350 nm, and still more preferably 300 nm. The lower limit of the mean particle size of the filler is more preferably 60 nm, and still more preferably 80 nm. When the mean particle size of the filler does not fall within the above range, desired air holes may not be suitably formed.

The content of the filler in the resin layer 3 is not particularly limited, and the content is preferably no less than 1 part by weight and no greater than 20 parts by weight with respect to 100 parts by weight of the resin as the principal component of the resin layer 3. The upper limit of the content of the filler is more preferably 15 parts by weight, and still more preferably 10 parts by weight. Whereas, the lower limit of the content of the filler is more preferably 3 parts by weight, and still more preferably 5 parts by weight. When the content of the filler does not fall within the above range, the desired air holes may not be suitably formed.

In the embodiments described above, the explanation has been made on the resin layer having air holes formed thereon; however, in the present invention, the resin layer may have valves of the air holes described above.

The valves of the air holes are formed so as to cover the air holes respectively, and are formed by adjusting the application amount when the coating liquid of the resin layer is applied. For example, when the resin layer is formed through applying the coating liquid with a roll having an intaglio on the surface thereof, the application amount is adjusted for the application such that a small amount of the coating liquid is applied at sites where cells of the gravure roll fail to abut, and that the air holes are formed at the sites on which a small amount of the coating liquid is applied and resin films that cover the air holes, respectively, are formed. An appropriate application amount allows the air holes to be formed on/through the resin layer, and the coating liquid surrounding the air holes joins above the air holes thus formed, whereby thin film valves are formed above the air holes. Accordingly, the valves also enable the permeation of oil to be prevented, whereas in regard to the release of the water vapor, the thin film valves are pressed by the pressure from the water vapor to generate gaps between the air holes and the valves, respectively; therefore, the water vapor is suitably released outside from the gaps.

It is to be noted that in order to form the valves of the air holes, the resin layer may be laminated such that the valves of the air holes are formed not only through adjusting the application amount, but also through adjusting the speed of rotation of the gravure roll, the components of the coating liquid, and the temperature and time period in drying the coating liquid, and the like.

Also, as the paper substrate, an oil-resistant paper may be used. The oil-resistant paper which may be used is any of well-known oil-resistant papers. Accordingly, the oil resistance of the microwaveable sheet can be improved.

Moreover, the material for forming the resin layer is not limited to those exemplified in the above embodiments, and any oil resistant resin is acceptable.

Although providing a printing portion is described in the above embodiments, the printing portion is not essential constituent feature according to the present invention. Also, in providing the printing portion, the modes are not limited to those in the above embodiments, and for example, a microwaveable sheet in which the printing portion is provided on one face of the paper substrate and the resin layer is laminated on the printing portion is also involved in the scope intended by the present invention.

Still further, the method for producing the microwaveable sheet in which the printing step is carried out after the resin layer-forming step is explained in the above embodiments, but the present invention is not limited thereto, and the printing step may be carried out before the resin layer-forming step S1. In addition, it would be possible to skip the printing step by providing a previously printed paper substrate beforehand.

EXAMPLES

Next, two Examples of the microwaveable sheet are explained, but the present invention is not limited thereto.

Example 1

As the paper substrate, an oil-resistant paper (trade name “EC grease-proof paper 25”: manufactured by Tenma Special Paper Manufacturing Co., Ltd.) having a grammage of 25 g/m² was used. A coating agent (trade name “Harvill B-7”: manufactured by Daiichi Toryo MFG., Ltd.) was diluted in water to prepare a coating liquid, and gravure coating of the coating liquid was carried out on one face of the paper substrate to form a resin layer. Thus, a microwaveable sheet of Example 1 was obtained having a plurality of air holes on/through a resin layer.

Example 2

The coating liquid was similarly applied also on another face of the microwaveable sheet in Example 1, and the coating liquid was hardened with drying to obtain a microwaveable sheet of Example 2.

Comparative Example

In Comparative Example, a thin Paper (trade name “HS21 paper”: manufactured by Oji Paper Co., Ltd.) on which polyethylene having a thickness of 8 μm was laminated was used.

Oil Resistance

An oil shieldability test was performed on Example 1 and Example 2, and Comparative Example. The shieldability test was conducted through determining a time period required for passage of 3 g of salad oil through the resin layer after dropwise addition, under a condition of a temperature of 40° C.

In Example 1, the oil did not pass through even after lapse of at least 3 hours. In Example 2, the oil did not pass through even after lapse of at least 5 hours. On the other hand, in Comparative Example, the passage of the oil occurred within 30 min.

Water Vapor Transmission Rate

Next, a water vapor transmission rate test was performed on Example 1 and Example 2, and Comparative Example. The water vapor transmission rate test was conducted in accordance with an upright cup test (JIS Z0208).

In Example 1, the water vapor transmission rate was 12,400 g/m²/24 hrs. In Example 2, the water vapor transmission rate was 8,500 g/m²/24 hrs. On the other hand, in Comparative Example, the water vapor transmission rate was 96 g/m²/24 hrs.

Results

Example 1 and Example 2 revealed results indicating a high water vapor transmission rate, accompanied by the oil resistance falling within a suitable range as described above. It is to be noted that in Comparative Example, the water vapor transmission rate was low, and thus an attachment of water droplets onto the inner face of the sheet was caused as a result of the heating in a microwave oven.

INDUSTRIAL APPLICABILITY

As described in the foregoing, the microwaveable sheet of the present invention, and the microwaveable sheet produced by the method for producing the microwaveable sheet of the present invention have oil resistance, and also have moisture permeability; therefore, the microwaveable sheets can be suitably used as a sheet for wrapping when foods are subjected to a heat treatment in a microwave oven or the like.

EXPLANATION OF THE REFERENCE SYMBOLS

-   1 microwaveable sheet -   2 paper substrate -   3 resin layer -   3 a air hole -   4 printing portion -   13 coating liquid -   14 gravure roll -   14 a cell -   15 vessel -   21 microwaveable sheet -   22 paper substrate -   23 resin layer -   23 a air hole -   34 solid roll 

1. A microwaveable sheet comprising: a paper substrate; and a resin layer laminated through applying a coating liquid of a resin composition on one face of the paper substrate, wherein the resin layer comprises a plurality of air holes.
 2. The microwaveable sheet according to claim 1, having a water vapor transmission rate of no less than 200 g/m²/24 hrs and no greater than 15,000 g/m²/24 hrs.
 3. The microwaveable sheet according to claim 1, wherein the air holes are formed through applying the coating liquid on the one face of the paper substrate with a roll having an intaglio on a surface thereof.
 4. The microwaveable sheet according to claim 1, wherein the air holes are formed through applying the coating liquid on the one face of the paper substrate having a shape with fine irregularity.
 5. The microwaveable sheet according to claim 1, wherein an application amount of the resin composition is no less than 0.01 g/m² and no greater than 20 g/m² on a basis of a solid content.
 6. The microwaveable sheet according to claim 1, wherein the resin layer is laminated on both two faces of the paper substrate.
 7. The microwaveable sheet according to claim 1, wherein a principal component of the resin layer is an acrylic resin, an olefin-derived resin or a carbonyl group-containing resin.
 8. The microwaveable sheet according to claim 1, wherein the paper substrate is an oil-resistant paper.
 9. The microwaveable sheet according to claim 1, further comprising a printing portion provided on one face of the paper substrate.
 10. The microwaveable sheet according to claim 1 having an oil shieldability from one side to another of no less than 1 hour.
 11. A method for producing a microwaveable sheet comprising: applying a coating liquid of a resin composition on one face of a paper substrate to form a resin layer, wherein the resin layer is formed such that a plurality of air holes are provided on/through the resin layer. 